A steam engine is an engine which uses steam from boiling water to make it move. The steam pushes on the engine parts to make them move. Steam engines can power many kinds of machines including vehicles and electric generators.
Steam engines were used in mine pumps starting in the early 18th century and were much improved by James Watt in the 1770s. They were very important during the industrial revolution where they replaced horses, windmills and watermills to work machines.
The first steam engines were piston engines. The steam pressure pushed on a piston which made it move along a cylinder and so they had a reciprocal (back-and-forth) motion. This could move a pump directly or work a crank to turn a wheel and work a machine. They operated at low pressure and had to be very big to make much power.
Steam engines were used in factories to work machines and in mines to move pumps. Later smaller engines were built that could move railway locomotives and steam boats.

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